WISWAG error

I'm running WebInspect 16.20 on a device that also has a REST interface that is (supposed to be ) based on swagger 2.0. I've tried running WISWAG.exe utility on the REST interface using the instruction in the help file:

     wiswag.exe -i <REST API definition URL>> -m <ResultingMacroName>

but I get an error:

"unexpected character encountered while parsing value:  h. Path '', line 0, position 0.

which looks like it didn't like the first character of my file.  Any other debug or verbose options i can use to figure out what's going on here?